This invention relates generally to nuclear reactors and more particularly to a recirculation system for use in pressurized water nuclear reactors to increase the outlet temperature of the reactor coolant.
The peripheral fuel assemblies of an open-lattice nuclear core characteristically operate at power levels significantly below that of the core average. The reactor coolant exiting from these peripheral fuel assemblies is at a temperature considerably below that of the rest of the core, and results in a degradation of the average coolant outlet temperature. For economic and environmental reasons, it is desirable to have the outlet temperature of the reactor coolant be as high as possible for a given size of reactor. For example, for each five degree fahrenheit increase in coolant outlet temperature for a 3800 megawatt thermal plant, the electrical output of the plant will increase by 6000 to 7000 kilowatts. This increase, in turn, reduces the size of reactor required, increases thermal efficiency, and reduces the amount of rejected thermal energy, with corresponding decreases in cost and environmental impacts.
Various methods have been utilized for attempting to increase the outlet temperature from the peripheral fuel assemblies, but none has proven entirely effective. Regional orificing, either at the inlet or the outlet, fails to provide the desired increase in temperature, because of the large amount of cross-flow of reactor coolant in an open-lattice core. Flow simulations have determined that the effect of the cross-flow in open-lattice cores results in a uniform reactor coolant flow less than halfway up the fuel assemblies.
Distributive orificing, wherein physical separation devices, such as orifice plates or support grids, are inserted at several locations along the peripheral fuel assemblies to provide a less-favorable flow path, are generally disfavored. Because the fuel assemblies in the center of the core burn up at a rate greater than those of the periphery, a common practice in the nuclear reactor field has been to shuffle the fuel assemblies; that is, to prolong the life of a given nuclear core, during the life of the core, the peripheral fuel assemblies are inserted near the center of the core, and the fuel assemblies which were located at the center of the core are removed from the reactor. Because of the additional distribution devices inserted into a fuel assembly for distributive orificing, the fuel assemblies are not similar, heat transport capability is reduced, and shuffling cannot occur. Thus, the increase of economy realized by the higher plant efficiency is lost by the inability to shuffle the fuel assemblies.
Changing the hydraulic diameter of the outer assemblies, or placing the peripheral fuel assemblies in a "can" to provide a closed flow channel, have the same drawbacks in that shuffling of the various fuel assemblies is prohibited.
Another important consideration in the design of nuclear reactors is safety. The nuclear reactors must be designed such that in the event of an accident, however unlikely it may be, the public is not thereby injured. One of the most serious accidents hypothesized is a double-ended pipe rupture, wherein one of the main coolant pipes breaks, resulting in a loss of coolant flow to the core and leading to a core meltdown. Prior practice in the field has been to store a large quantity of coolant for use in emergencies, and connect this stored coolant to the main coolant inlets of the reactor pressure vessel. Then, in the unlikely event of a pipe rupture, the stored coolant was pumped at high pressure to the main coolant inlet, where it flowed down to the bottom of the pressure vessel. Two problems are associated with this solution.
In this type of accident, the coolant already in the nuclear reactor pressure vessel heats, changing to steam, and this steam gathers in the downcomer annulus between the core barrel and the pressure vessel where the emergency coolant is attempting to flow. This steam seriously impedes a flow of the emergency coolant, thereby causing the emergency injection to lose much of its effectiveness.
The other problem is that, if the pipe rupture accident occurs in one of the coolant inlet pipes, the emergency coolant which is being pumped to that inlet nozzle will, instead of entering the pressure vessel to flood the nuclear core, flow through the pipe and out through the rupture.